1. Field of the Invention
The invention relates to a system for monitoring the operation of cages moving in mine shafts. More specifically, the invention relates to such a system for monitoring the speed of cages moving in such mine shafts wherein said speed is detected using digital means and analog backup means, and the digitally derived speed value is compared with the analog derived speed value to determine if the digital means are operating correctly.
2. Description of Prior Art
The first hoist controllers were mechanical and were referred to .as the so-called Lily controllers. These had the problems of backlash, being difficult to adjust, and not being easy to test. In addition, if any element failed, then the entire system would fail as described by Roger Davies in Automated Hoists Britain, Post-Markham, published in World Mining Equipment, March 1989, at pages 29 to 36.
Accordingly, the mechanical systems were replaced by electronic systems, and examples of electronic systems are described in the Davies article. Basically, the electronic systems consisted of toothed wheels and pulsers which would sense the passing of the toothed wheels and provide output pulses each time a toothed wheel passed the pulsor. The toothed wheel could be connected to the shaft of the drum of the hoist system so that the pulsers would have indications of the position of the cage in the mining shaft. The position signal as determined by the pulsers was compared with a position signal as determined by proximity switches in the shaft (see FIG. 2 of the article) or by the position signal as determined by pulses from magnetized rope (see FIG. 4 of the article). Both the proximity switches and the magnetized rope systems are expensive and not very reliable.